Dalton's
Law - for "perfect gases,"
a mixture of gases will have a pressure equal to the sum of the pressures
of the individual gases, assuming no chemical reaction has taken place
between the gases. Named for John Dalton (1766 - 1844), a British chemist
who formulated the concept.

dBZ
- the nondimensional "unit" of radarreflectivity. It represents a logarithmic
power ratio (in decibels, or dB) with respect to radar reflectivity factor,
Z. The value of Z is a function of the amount of radar beam energy
that is backscattered by a target and detected as a signal (or echo).
Higher values of Z (and dBZ) thus indicate more energy being backscattered
by a target. The amount of backscattered energy generally is related
to precipitation intensity, such that higher values of dBZ that are detected
from precipitation areas generally
indicate higher precipitation rates.

Degree
Day - a measure of the difference between the mean
daily temperature and some given base temperature: one degree day
is given for each degree (degree Celsius
or degree Fahrenheit) of departure above
(or below) the base temperature during one day

Density
- the mass of a substance per unit volume

Desertification
- a tendency toward more prominent desert conditions in a region

Dew
- water condensed upon the
surfaces of objects near the ground when temperatures
of the surface air have fallen below the dew point
due to cooling during the night but are still above freezing

Dewpoint
Depression - the difference in degrees between the air temperature
and the dewpoint temperature

Differential
Motion - specifically, cloud motion that appears to differ
relative to other nearby cloud elements; cloud rotation is one example
of differential motion, horizontal wind shear
along a gust front is another example

Difluence- a pattern of wind flow in which air moves outward (in
a "fan-out" pattern) away from a central axis that is oriented parallel
to the general direction of the flow; opposite of confluence;
difluence is not the same as divergence

Direct
Solar Radiation - the component of solar radiation received
by the earth's surface only from the direction of the sun's disk (i.e.
it has not been reflected, refracted or scattered)

Directional
Shear - the component of wind shear
which is a result of a change in wind direction, e.g., southeasterly winds
at the surface and southwesterly winds aloft

Diurnal
- daily; related to actions which are completed during a single
calendar day, and which typically recur every calendar day (e.g., diurnal
temperature cycle of temperature increase and decrease)

Divergence
- the net outflow of air from a region, typically caused by
horizontal wind motion; the opposite of convergence

Doppler
Dilemma - a limitation with a pulsed Doppler
radar (like the WSR-88D) that involves a trade-off between a wide
range of observable radial velocities
and the detection of echoes at a long range
from the radar. Having a wide range of velocities (desired for detection
of severe weather) limits the range from the radar that echoes can be
detected. When more areal coverage (e.g., a long range) is desired, a
narrower range of radial velocities must be computed. The Doppler Dilemma
is related to the time between successive transmitted pulses of energy.
A "long" amount of time (in milliseconds) between successive pulses allows
the radar to detect echoes at a far range from the radar. However, a short
amount of time between successive pulses allows for more accurate and
higher Doppler velocities to be calculated.

Dry
Microburst - a microburst with
little or no precipitation reaching
the ground; most common in semi-arid regions; at the ground, the only
visible sign might be a dust plume or a ring of blowing dust beneath a
local area of virga

Dryline
(or Dry Line) - a boundary separating warm, dry air
from warm, moist air, typically across parts of New Mexico, Texas, Oklahoma,
or Kansas. It typically lies north-south across the central and southern
high Plains states during the spring and early summer, where it separates
moist air from the Gulf of Mexico (to the east) and dry desert air from
the southwestern states (to the west).

Dry-Line
Bulge - a bulge in the dry line, representing
the area where dry air is advancing most strongly at lower levels. Severe
weather potential is increased near and ahead of a dry line bulge.